Fuel cell systems are used for the conversion of energy from liquid and gaseous fuels into electrical current and heat. The conversion of energy takes place quietly in the fuel cell and at an efficiency ratio of between 50% and 60% during the conversion of chemical into electrical energy, depending on the selected current density in the cell. The advantages over an engine-driven current generator are particularly effective in the kW range in the case of small outputs as are required for the net-remote power supply and for decentralized power-heat coupling. Consequently, work to develop fuel cell systems is done worldwide, however, until now without any break-through on the market.
The reasons therefor are mainly the high manufacturing costs for the complete, highly complex system. This system comprises fuel processing (using a so-called fuel processor) with a reformer and the fuel cell stack as well as with peripheral components such as heat exchangers, pumps, valves and electrical apparatus for automatic operation.
The Polymer Electrolyte Membrane (PEM) fuel cells that are equipped with polymer membranes and designed for operating temperatures up to approximately 80° C. require carbon monoxide, CO, fine-scrubbing of the reformate up into the parts per million, ppm, range and expensive water management for the humidification of the cathode air. In this case, the process water for the steam reformer may not be evaporated with the exhaust heat of the stack, because the temperature is too low for this.
Document WO 2005/084771 A2, or U.S. Patent Application Publication No.: US 2007/0006592 published Jan. 11, 2007 in the same patent publication family, describes a compact reformer with an integral evaporator. This can be used for reformation with an efficiency ratio of up to 80%. The resultant electrical efficiency ratio for the entire system is 35 to 40% if losses of 10 to 15% of the gross power generation due to auxiliary assemblies such as pumps, blowers and current transducers are taken into account.
Considering high-temperature PEM cells (publication by PEMEAS und Pat) that have recently become available and that operate at temperatures of 120 to 200° C., the CO fine-scrubbing and the water management may be omitted, thus permitting a considerable simplification of the process. In addition, the exhaust heat of the stack may be used for the evaporation of the process water.
The object of the invention is to further simplify the overall process on the basis of high-temperature cells in order to lower the manufacturing costs and to provide a safer automatic operation. In so doing, the electrical efficiency ratio of the total system should not drop below the level of 35 to 40% and, if possible, be even higher.